The present invention relates to a common electrode drive circuit and a liquid crystal display.
At present, LCDs (Liquid Crystal Displays), especially TFT-LCDs (Thin Film Transistor-Liquid Crystal Displays), are increasingly used by virtue of their lightness, slimness, portability and etc. However, flickering images often occur in conventional LCDs in use, which affects display quality of the LCDs. Below a brief explanation to the generation of flickering images in a LCD is given.
A LCD comprises a plurality of pixels arranged in a matrix. FIG. 1 is a schematic diagram of an equivalent circuit for each pixel in a LCD. As shown in FIG. 1, when a TFT-LCD is in operation, on an array substrate, a gate switching-on (“ON”) voltage is applied to a gate electrode g connected with a gate line Gn, to turn on the TFT, so that a data voltage for displaying image on a data line Dm is applied onto a drain electrode d through a source electrode s. The drain electrode d is connected with a pixel electrode p, and thus the above-mentioned data voltage is applied onto the pixel electrode p through the drain electrode d to generate a pixel electrode voltage. A common electrode layer is provided on a color filter substrate, and a liquid crystal capacitor Clc is created between the pixel electrode p and the common electrode layer on which a common voltage Vcom is applied. The liquid crystal capacitor Clc exerts an electrical field on liquid crystal molecules to orientate the liquid crystal molecules. In order to prevent liquid crystal material from deterioration, the pixel electrode voltage may be reversed with respect to the common voltage, so as to drive the deflection of liquid crystal material with a reverse driving method in which the driving voltage is switched between the positive and negative values repeatedly, to control transitivity of light and display images of different grey levels. During reverse driving, if it is desired to make grey levels for an image and its reversed image to be consistent, differences between the pixel electrode voltage and the common voltage Vcom for the image and its reversed image have to be close to each other in absolution value. Otherwise, flickering images will occur.
As a parasitic capacitor Cgd is generated between the gate electrode g and the drain electrode d, obvious fluctuation of voltage generated when the gate line Gn is switched on and off will be applied to the pixel electrode p through the parasitic capacitor Cgd, causing a voltage jump ΔV in the pixel electrode voltage and affecting the precision of the eventual pixel electrode voltage.
FIG. 2 is a schematic waveform diagram showing the change in the pixel electrode voltage. As shown in FIG. 2, when the gate line is turned off, the gate voltage Vg may have a relative large voltage drop of about 10˜40V, which will affect the pixel electrode voltage Vp through the parasitic capacitor to generate a voltage jump ΔV, and such influence will exist all along until the gate line is turned on next time. Therefore, the influence of this voltage jump on displayed grey level can be noticed by a human's eye. When the gate line is turned on next time, the data voltage Vd reverses in polarity, so that the gate line is turned off again, and the voltage jump ΔV will cause the new pixel electrode voltage Vp to drop too. Accordingly, the pixel electrode voltage Vp is lower than the data voltage Vd, and the value by which the voltage drops is exactly the value of the voltage jump ΔV which is caused by the change in the gate voltage Vg through the parasitic capacitor. Thus, the phenomenon of flickering images occurs.